Builds on #72787's current_root guard (cherry-picked with authorship
preserved). Two further hardenings for #72776:
- require the settled cwd and the workspace to share the same common .git
dir (the shape 'git worktree add' produces), so a git workspace visiting
an UNRELATED repo is a browsing visit, not a re-home (repro'd by
Johnny-xuan in the issue thread);
- never reconcile over an explicitly chosen workspace (explicit_cwd),
while a settle-adopted cwd stays followable via a cwd_from_settle
marker cleared by _set_session_cwd / project switch.
Fixes#72776
8babfe95b4